It ____ be our headmaster. He has gone to Beijing.

Helen _________ go on the trip with us, but she isn’t quite sure yet. 

—Can I pay the bill by check?  —Sorry, sir. But it is the management rule of our hotel that payment ____ be made in cash. 

Without the ‘ greenhouse effect’, the earth ____ be about thirty-three degrees Celsius cooler than it is.

Since nobody gave him any help, he    have done the research on his own.